Methods for processing plastic, especially thermoplastics, to make personal and consumer items and packaging typically utilize one or more polymers and employ techniques such as injection molding, blow molding, extrusion, and thermoforming. Microcellular techniques are employed to disperse gases in the polymer, thereby resulting in the polymer being “foamed.” The “foamed” polymer includes a preset amount of dissolved gas that, when heated and processed, emerges from the plastic item or packaging in the form of bubbles or voids.
Microcellular injection molding is an emerging special injection molding process capable of producing foamed parts with many advantages. During the microcellular injection molding process, a supercritical fluid is introduced into a molten polymer prior to the polymer being injected into a mold. The polymer solidifies in the mold to form a desired component. The introduction of the supercritical fluid prior to injection of the polymer into the mold causes tiny bubbles to be distributed throughout the molded component. By providing tiny bubbles in the molded component, the amount of material necessary to mold the component is reduced, while the dimensional stability of the molded component is improved. Hence, this microcellular injection molding process allows for the production of lightweight and dimensionally stable plastic components with complex geometries while reducing the amount of raw material.
While the microcellular injection molding process saves on material cost and improves production efficiency as compared to conventional solid injection molding, the process does have certain limitations. By way of example, microcellular injection molding requires specially designed supercritical fluid delivery and dosing systems to be installed on the injection molding machine for the delivery of the supercritical fluid as a physical blowing agent. In addition, modifications need to be made to the injection molding machine itself, including the installation of a supercritical fluid delivery device and a special injection screw with mixing elements for effectively mixing the supercritical fluid with the liquid polymer. These two factors lead to an increase in capital investment, especially when a large number of injection molding machines need to be modified.
Alternatively, other methodologies may be used to produce foamed molded parts. By way of example, chemical blowing agents can be used to produce foamed injection molded parts without the need of installing any additional equipment on the injection molding machine. However, residuals tend to appear in the parts after the reaction, which leads to the degradation of the polymer matrix and to possible contamination of the mold. Furthermore, the use of chemical blowing agents does not allow for good control over the foaming process or the cellular foam structure of the parts. In addition, chemical blowing agents are not suitable for processing high temperature polymers due to its early decomposition.
A still further way to produce, foamed injection molded parts using a conventional injection molding machine is to saturate pellets of a polymer with a physical blowing agent in a high pressure vessel prior to introduction of the polymer into the injection molding machine. While parts with a cellular foam structure can be manufactured utilizing this methodology, the production rate of pre-saturated pellets using a high pressure vessel is usually not high enough for continuous mass production of injection molded parts.
Therefore, it is a primary object and feature of the present invention to provide a method for fabricating foamed, injection molded components in a cost-effective way while ensuring a satisfactory production rate.
It is a further object and feature of the present invention to provide a method for fabricating foamed injection molded components which produces components having smaller cell size and increased cell density over components produced by current methods.
It is a still further object and feature of the present invention to provide a method for fabricating foamed, injection molded components which produces lightweight components with comparable properties as those produced by current methods, but at a lower cost.
It is a still further object and feature of the present invention to provide a method for fabricating foamed, injection molded components which is simple and which may be performed with standard injection molding machinery.
In accordance with the present invention, a method of fabricating an injection-molded component is provided. The method includes the step of introducing pellets including a first supercritical fluid and polymeric material into an injection barrel of an injection molding machine. The pellets are plasticized within the injection barrel and a second supercritical fluid is introduced into the plasticized pellets to form an injection material. The injection material is injected into a mold.
The method may include the additional steps of providing a polymeric material. The polymeric material is heated and the first supercritical fluid is introduced to produce a melt. The melt is extruded and cooled rapidly to solid strands. The strands are then pelletized. The extruded melt is unfoamed. It is contemplated for the first supercritical fluid to be nitrogen and for the second supercritical fluid to be carbon dioxide. The injection material may be mixed prior to injecting the injection material into the mold. The injection molding machine includes a hopper communicating with the injection barrel. The pellets are introduced into the hopper prior to introduction of the pellets into the injection barrel.
In accordance with a further aspect of the present invention, a method of fabricating an injection-molded component is provided. The method includes the steps of providing a polymeric material and introducing a first supercritical fluid into the polymeric material. Pellets are formed from the polymeric material and the first supercritical fluid. The pellets are introduced into an injection barrel of an injection molding machine and the pellets are plasticized. A second supercritical fluid is introduced into the plasticized pellets to form an injection material. The injection material is injected into a mold.
The step of forming pellets from the polymeric material and the first supercritical fluid may include the additional steps of heating the polymeric material and introducing the first supercritical fluid to produce a melt. The melt is extruded and the cooled rapidly to form solid strands. The strands are introduced into a pelletizer. The extruded melt is unfoamed. It is contemplated for the first supercritical fluid to be nitrogen and the second supercritical fluid to be carbon dioxide.
The supercritical fluid is mixed into the plasticized material prior to injecting the injection material into the mold. The injection molding machine includes a hopper communicating with the injection barrel. The pellets are introduced into the hopper prior to introduction of the pellets into the injection barrel.
In accordance with a still further aspect of the present invention, a method of fabricating an injection-molded component is provided. The method includes the step of introducing pellets into an injection barrel of an injection molding machine. The pellets include a first supercritical fluid. The pellets are plasticized in the injection barrel and a second supercritical fluid and the plasticized pellets are mixed to form a mixed material. The mixed material is injected into a mold.
The pellets are formed from a polymeric material and the first supercritical fluid. The step of forming pellets from the polymeric material and the first supercritical fluid includes the steps of heating the polymeric material and introducing the first supercritical fluid to produce a melt. The melt is extruded and cooled to form solid strands. The strands are introduced into a pelletizer. The extruded melt is unfoamed. The first supercritical fluid may be nitrogen and the second supercritical fluid may be carbon dioxide. The injection molding machine includes a hopper communicating with the injection barrel. The pellets are introduced into the hopper prior to introduction of the pellets into the injection barrel.